1. FIELD OF THE INVENTION
The present invention relates to a dielectric dispersion determining method in a terahertz region using a femtosecond ultrashort visible optical pulse for determining a dielectric constant (and/or refractive index) of a nonlinear optical crystal in a terahertz region with a femtosecond ultrashort visible optical pulse.
2. DESCRIPTION OF THE RELATED ART
In a wavelength conversion technique called parametric conversion using a nonlinear optical crystal, visible rays are converted into far infrared rays with frequencies over the range of 0.1-100 terahertz (wavelengths in the range of 3 mm to 3 micron). Dielectric constants (and refractive indices) in the far infrared region are important optical constants indicating the performance of crystals in searching for a new crystal system or in establishing phase matching conditions providing most effective conversion.
Conventionally, the terahertz region dielectric constants (refractive indices) are obtained by measuring reflectances or transmittances using a far infrared optical measuring system. Such a system, however, requires a light source like a far infrared lamp, an optical waveguide device or optical focusing system, and a detector, which are special devices and thus expensive. In particular, the detector needs delicate handling such as cooling with cryogen.
Such a direct method, however, is not essential in obtaining the dielectric constants (refractive indices) in the far infrared region. For example, they can be acquired in principle by reading with visible rays changes induced by infrared rays through electrooptic effect in the refraction area in the visible region. This becomes feasible by a method of solid state measurement called "time domain measurement" growing with the progress in the femtosecond (10.sup.-15 second) ultrashort visible optical pulse generating technique over the last 15 years.
In a transient grating method, one of fundamental measuring techniques using a ultrashort visible optical pulse, two ultrashort visible optical pulses are crossed in a sample, thereby generating sample excitation with a spatially periodic structure, that is, a grating. A third ultrashort pulse applied thereto will be diffracted by the grating. Thus, changes of the grating with time can be obtained by recording the intensity of the diffracted light with varying the incident time of the third ultrashort pulse through a delay optical path. A transient response thus obtained has not been used for determining the dielectric constant (refractive index) because the conventional interpretation thereof places emphasis on the dependence of the infrared ray frequency on the wavelength (grating constant).
Although the conventional interpretation of the transient response in the transient grating method places emphasis on the dependence (dispersion relation) of the infrared rays on the wavelength (grating constant), an ambiguity in the definition of the dispersion relation makes it difficult to determine the dielectric constant (and/or refractive index) based on the dispersion relation.